Complex behaviors such as aggression are the result of interactions between external stimuli from the environment and the internal activity of the brain. Some individuals are very aggressive while others are very passive. Even though this is in large measure context dependent, there is a clear genetic component that is involved in aggression in all known species. Because aggression occurs widely in the animal kingdom it is possible that the genetic mechanisms that cause this behavior are similar in all species and derive from their common evolutionary ancestry. We study aggression in Drosophila melanogaster, and have developed simple test systems to measure the amount of fighting behavior between male flies. We also use an automated analysis method as an independent measure of aggression. Recently, we have identified a transcriptional control module that appears to be remarkably conserved between flies and mammals. We identified two transcriptional repressors that regulate the activity of neurosecretory cells in the adult fly brai affecting the release of neuropeptides. In addition to this transcriptional regulation mechanism, neuromodulatory control of aggression is another critical mechanism in the control of aggressive behavior. This proposal focuses on the identification of a minimal circuit that is crucial for the modulation of aggressive behavior. We have previously shown that aggression in flies is strongly affected by serotonin (5-HT) modulation. Here, we propose to identify the serotonin receptor that is necessary and sufficient for serotonergic modulation of aggression. We will also identify the critical set of receptor expressing neurons that receive the serotonin signals and those that produce it. This will establish the minimal circuit that is necessary and sufficient for serotonergic modulation of aggression. This research will help understand how a complex set of neuromodulatory neurons that affect a wide range of physiological and behavioral responses is capable of specificity of these different responses through specific subcircuits with specific receptors.